In surveying instruments, especially in so called total stations, it is required to position a movable unit in the instrument around a horizontal axis and a vertical axis by rotating the movable unit around the axis required. The rotation of the movable unit around either axis is normally effected via a gear arrangement either by hand for a manual instrument, or by means of an electric motor connected to the gear arrangement for an automated instrument that is intended to automatically move from one position to another in a procedure of tracking, i.e. in which a total station locks on and follows a target or finds a specific target in a known position. Such gear arrangements need to be of very high precision in order to serve their purpose, and are as a consequence quite expensive. In such a gear arrangement there is generally a mechanical backlash and a hysteresis effect, leading to a need for angle sensors both on the motor and on the movable unit in order to obtain a sufficiently stable and exact servo loop for controlling the motor.
The presence of a gear arrangement with high gear ratio in an apparatus that should allow forced movement by hand of the movable unit, typically requires a friction clutch between the motor and the movable unit, allowing the movable unit to move while the gear arrangement remains in position.
Motor drive systems of the above type are expensive due to the complexity of details, with high demands on tolerances for the gear wheels. Gear wheels automatically imply wear and tear, lubrication requirements, service intervals etc.
Motor drive systems used for these purposes in the prior art have exhibited high gear ratio (typically 600 times in a total station) and this inherently means that there will be some play present. The common motor drive used in the prior art requires two rotation sensors, one on the motor axis giving a fast but not so accurate angle measurement, and one on the outgoing axis giving precision adjustment information.
Examples of motor drive systems for surveying instruments may be found in U.S. Pat. No. 5,987,763 and U.S. Pat. No. 5,475,930. A brushless DC permanent magnet motor is known from WO99/03188 (PCT/US98/14010) and a method for making a motor winding from U.S. Pat. No. 5,197,180. The disadvantage of using a gear has already been discussed above. The winding method described in U.S. Pat. No. 5,197,180 comprises the step of reversing the winding direction for each group of windings, which may cause complications in the winding.